Dredge spoil disposal presents a significant problem for dredging contractors. The dredged sludge is suspended in water in the form of a slurry. Such slurries may create serious recovery problems if discharged directly into holding lagoons or settling basins. Accordingly, there is a need to efficiently dewater and reclaim the solid materials from these slurries. This invention provides a quick and efficient means for the separation of water from solids, thereby economically reducing volumes and thereby introducing an attractive option for sediment recovery, re-use, or disposal.
Hydraulic dredges are effective excavation devices for removal of a wide variety of sediments from natural or manmade waterways. The dredges may remove sediments which are classified as contaminated or hazardous sediments as well as non-hazardous sediments. The sediments may comprise debris such as sand, gravel, clays, silts, organic matter, or any combination thereof. Typically, the finest fraction of solids, including clays, silts, and organic matter, contributes the greatest volume and this fraction is also the most difficult to recover. All of the materials excavated from a waterway in the hydraulic dredging process must be removed to a disposal site. Traditionally, these sites include settling basins specifically engineered to accommodate the slow settling characteristics of the finest of the particulate matter. These “sludge lagoons” burden the immediate community in various ways. They occupy significant areas of land and are generally unpleasant neighbors because of the noxious odors emitted during the slow process of drying out the hydrated sediments.
In an attempt to solve the problems associated with such traditional settling basins, dredge slurry dewatering systems were introduced. These dewatering systems are used to separate dredge solids from water so that the solids may be recycled or disposed. The systems typically comprise screening assemblies, hydrocyclones, centrifuges, belt presses and clarifying vessels. Most of these systems operate as “batch treatment” processes, which can accept only defined quantities of dredge spoil at spaced intervals. The dredge spoil is first pumped into a holding vessel, then, while the dredge sits idle, the slurry is slowly pumped and treated by each apparatus in turn, until the holding vessel can be refilled. The processing rates of the individual equipment components of the dewatering system thus limit the throughput rate of the dredging operation. The inefficiencies associated with such batch systems serve to raise the costs of operation.
Continuous de-watering systems have been proposed by the inventor. In general, such systems may be described as developments of a generic slurry dewatering system and can be seen to provide apparatus for de-watering a stream of slurry comprising a first separator, for separating objects above a first size from a stream of slurry, to produce a stream of primary treated slurry, a second separator for receiving the stream of primary treated slurry and for separating objects above a second size from the stream of primary treated slurry to produce a stream of secondary treated slurry, the second size being smaller than the first size and a third separator for receiving the stream of secondary treated slurry and for separating objects above a third size from the stream of secondary treated slurry, the third size being smaller than the first and second sizes.
Further levels of treatment may be provided, for example further removal of solids from the treated water or further dewatering of the solids removed by earlier stages from the stream of slurry.
U.S. Pat. No. 5,656,174 (“Hodges '174”), issued on 12 Aug. 1997 to one of the inventors herein, describes a dewatering system that is capable of continuous operation. The system employs a first separator in the form of a series of filtration screens, and optionally a second separator such as a hydrocyclone, followed by addition of a flocculating agent to aggregate the fine solids. The flocculated material is then filtered by passage over a third separator in the form of a tracking screen assembly having two screens supported on an A-shaped frame. Optionally, the treated water is centrifuged to remove further solids.
U.S. Pat. No. 6,149,811 (“Hodges '811”), issued 21 Nov. 2000 to one of the inventors herein, discloses a screen device for collecting solids in a slurry feed stream, comprising a plurality of screens connected together at various angles. It can be used as the third separator
A PCT application WO01/85628 A1 (“Hodges PCT '628”), published 15 Nov. 2001, provides a sludge de-watering system that is capable of delivering high flow rates and which is easy operate. A patent family member of this application issued on 25 Nov. 2003 in the United States as U.S. Pat. No. 6,652,757 (“Hodges '757”) to one of the inventors herein. It may be applied particularly in the final stage of the process described in Hodges '174. A stream of slurry is de-watered by continuously determining one or more parameters, adding a flocculating agent to the stream of slurry in response to the determined parameters, mixing the flocculating agent with the slurry to afford a stream of flocculated slurry, delivering the flocculated slurry through a diffuser onto a tracking screen and filtering the flocculated slurry through the tracking screen to provide solids and a filtrate. The parameters are the flow rate of the slurry, the pulp density of the slurry and the pulp density of the filtrate. The tracking screen suitably provides a third separator for use in the apparatus of Hodges '174.
Hodges PCT '628 particularly discloses a diffuser which is designed to deliver of stream of flocculated slurry to a tracking screen, the diffuser comprising a vertically disposed conduit rising from a base having the inlet for the stream of flocculated slurry to an upper portion where the stream of flocculated slurry is delivered to the tracking screen, said conduit having a horizontal sectional area that remains constant or increases from the base to the upper portion and an aspect ratio that substantially increases from the base to the upper portion.
Hodges PCT '628 further discloses that the flocculated slurry removed by the tracking screen may further be dewatered by use of a solids belt press.